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An alien mussel first spotted in Lake Michigan a decade ago has colonized much of the lake bottom, creating problems that likely will surpass those caused by the dreaded zebra mussel.

Meet the quagga mussel, the tougher, more disruptive cousin of the zebra mussel. Imported to Lake Michigan in 1997 in the ballast water of ocean freighters, quaggas now blanket much of the lake bottom to depths of 330 feet, according to scientists at the National Oceanic and Atmospheric Administration. That's bad news for anglers because quaggas, like zebra mussels, are filter-feeders that eat some species of invertebrates and plankton at the base of a food web that supports salmon and whitefish. Changes caused by zebra mussels have been linked to decreasing numbers of alewife in Lake Michigan, which has led to smaller salmon and whitefish.

The quagga is like a frost-proof zebra mussel on steroids -- it's a heartier species that, unlike the zebras, can survive in cold water and live on the entire lake bottom. Zebra mussels thrive in the shallow, warmer waters of Lake Michigan and other inland lakes.

"I think quagga mussels are going to have a much larger ecological impact on Lake Michigan than zebra mussels ever did," said Gary Fahnenstiel, a NOAA senior ecologist and head of the agency's Lake Michigan Field Station in Muskegon. "From a historical perspective, the long-term impacts of quaggas are going to be right up there with the problems caused by sea lamprey and alewife."

NOAA scientists recently discovered quagga mussels covering vast areas of the lake bottom. The average density of quagga mussels at sites sampled last summer was 7,700 per square meter, up from 2,200 per square meter in 2002.

"I knew quagga mussels were starting to take hold in the lake in 2000, but by the time we sampled this past summer the numbers were just amazing. They've almost totally replaced zebra mussels," said Tom Nalepa, a research biologist at NOAA's Great Lakes lab in Ann Arbor.

"Not only are we seeing increased numbers of quagga mussels, their spatial distribution is increasing and they are going a lot deeper than zebra mussels, too," Nalepa said.

It is likely quaggas eventually will blanket all of Lake Michigan's sandy and muddy bottom, even in the deepest parts of the lake, Nalepa said.

"As these quagga populations increase, something has got to give in the lake ecosystem," Nalepa said. "This isn't good in terms of the lake's biodiversity or anything else."

Quaggas and zebra mussels intercept the microscopic food that supports diporeia, a tiny shrimplike creature that is a major food source for many Great Lakes fish. As the number of diporeia in the lake has dropped in recent years, the number of alewife has plummeted and whitefish and salmon have become smaller and skinnier.

"It's a huge issue; you can see the effects when you're out fishing," said Denny Grinold, a longtime charter fishing captain who owns and operates a boat in Grand Haven. "You can't fish on the bottom for lake trout or salmon -- you'll foul your gear with mussels."

Zebra and quagga mussels also clog the nets commercial and tribal fishermen use to catch whitefish and chubs in Lake Michigan, said Tom Gorenflo, tribal fisheries manager for the Chippewa-Ottawa Resource Authority in Sault Ste. Marie.

Gorenflo said the mussels also are suspected of causing an explosion in the growth of algae on the lake bottom. He said the algae often coats commercial fishing nets with "slime."

Grinold, past president of the Michigan Charter Boat Association, said he believes zebra and quagga mussels have caused almost as much harm to the Lake Michigan sport fishery as sea lamprey.

The eel-like sea lamprey decimated the lake trout fishery in the 1950s. As lake trout vanished, alewife became the dominant fish species in Lake Michigan by the 1960s. The state began stocking salmon in 1966 to reduce the number of alewife, which were dying in huge numbers and littering beaches.

The state's grand biological experiment with salmon stocking worked well until zebra mussels arrived in the Great Lakes in the late 1980s and profoundly changed the lakes' food web.

Zebra mussels filter huge quantities of water, about a quart per day for each mussel. The mussels increase water clarity by intercepting the microscopic species that other bottom-dwelling species, such as diporeia, need to survive.

Quaggas cause similar effects but on a much larger scale, Nalepa said.

Scientists believe zebra mussels consuming diporeia have contributed to the collapse of the alewife population in Lake Huron.

Some scientists and anglers fear Lake Michigan's alewife population could be the next to collapse, which would be disastrous for the salmon fishery; salmon dine almost exclusively on alewife. The spread of quagga mussels only will make life more difficult for alewife, whitefish and salmon in Lake Michigan, NOAA scientists said.